Amygdalar efferents initiate auditory thalamic discriminative training-induced neuronal activity.

It is well known that neurons of the medial geniculate (MG) nucleus of the thalamus send axonal projections to the amygdala. It has been proposed that these projections supply information that supports amygdalar associative processes underlying acquisition of acoustically cued conditioning and learning. Here we demonstrate the reverse direction of influence. Temporary inactivation of the amygdala using the GABA(A) receptor agonist muscimol just before the onset of discriminative avoidance conditioning permanently blocked the development of training-induced discriminative neuronal activity in the MG nucleus of rabbits. No discriminative activity developed when the amygdala was inactivated or during later training to criterion without muscimol. Thus, amygdalar processing at the outset of training is necessary for the development of training-induced discriminative activity of neurons in the MG nucleus.